1. Field of the Invention
The present invention relates generally to apparatus for sensing tilt, and more particularly to tilt sensors for magnetically sensing the tilt of an object.
2. Description of the Prior Art
Tilt sensors are used in a variety of applications to detect the angular orientation of an object with respect to a reference, usually supplied by gravity. Tilt sensors typically comprise a fixed element that is rigidly mounted to an object whose tilt is to be measured, a reference element that maintains a constant orientation with respect to gravity, and means for sensing the position of the reference element with respect to the fixed element.
Capacitive tilt sensors typically use conductive plates as part of the fixed element. A reference element such as a pendulum moves in or out of the space between the plates in response to a tilt, changing the permittivity of the space between the plates, and hence, the capacitance. Alternatively, the reference element may be a fluid that partially fills the space between the plates. The capacitance changes as the fluid-covered area of the plates changes. This change in capacitance is converted to a corresponding tilt angle. Since dielectric constants are greatly dependent upon ambient temperature, complex temperature compensation circuitry must be added for accurate tilt measurement in varying ambient temperatures.
Electrolytic tilt sensors are typically comprised of a nonconductive container that is partially filled with an electrolyte. At least three electrodes, including one common electrode, extend through the container so that at least a portion of each is immersed in the electrolyte. As the container is tilted, the electrodes become more or less immersed, causing a change in impedance between any one electrode and the common electrode. When the electrodes are configured as part of an appropriate electrical circuit, the angle of tilt can be correlated to an output voltage of the circuit. Problems with electrolytic tilt sensors include the fact that electrochemical reactions occur when a voltage is applied across an electrolyte. These reactions change the resistivity of the electrolyte, causing a deviation in the output voltage for a given tilt angle. Electrolytic tilt sensors therefore have relatively short lives. Response time is also limited due to the viscosity of the electrolyte, and the surface tension of the electrolyte can adversely affect resolution.
Magnetic tilt sensors typically comprise a permanent magnet housed in a nonmagnetic case. The magnet serves as the reference element, moving within the case in response to gravity. Magnetic sensors detect the position of the magnet within the case, and the tilt angle is determined therefrom. Numerous methods have been proposed for minimizing friction between the moving magnet and the surrounding case. Such methods have been unsatisfactory and often result in increased response times and decreased sensitivity.
Thus a need exists for a tilt sensor that has a simple structure, good response characteristics, and a long expected life.